It 's been a cruel irony for ancient mariners and any thirsty person who has ever gazed upon a sparkling blue ocean : Water , water everywhere , and not a drop to drink .

But imagine a coastal city of the future , say in 2035 . Along with basic infrastructure such as a port , roads , sewer lines and an electrical grid , it 's increasingly likely this city by the sea will contain a newer feature .

A desalination plant .

Thanks to improved technology , turning ocean water into freshwater is becoming more economically feasible . And a looming global water crisis may make it crucial to the planet 's future .

The United Nations predicts that by 2025 , two-thirds of the world 's population will suffer water shortages , especially in the developing world and the parched Middle East . Scientists say climate change is making the problem worse . Even in the United States , demand for water in drought-ravaged California and the desert Southwest is outpacing supply .

This is why a huge desalination plant is under construction in Carlsbad , California , some 30 miles north of San Diego . When completed in 2016 , it will be the largest such facility in the Western Hemisphere and create 50 million gallons of freshwater a day .

`` Whenever a drought exacerbates freshwater supplies in California , people tend to look toward the ocean for an answer , '' said Jennifer Bowles , executive director of the California-based Water Education Foundation . `` It is , after all , a seemingly inexhaustible supply . ''

A growing trend

Most desalination technology follows one of two methods : distillation through thermal energy or the use of membranes to filter salt from water .

In the distillation process , saltwater is heated to produce water vapor , which is then condensed and collected as freshwater . The other method employs reverse osmosis to pump seawater through semi-permeable membranes -- paper-like filters with microscopic holes -- that trap the salt while allowing freshwater molecules to pass through . The remaining salty water is then pumped back into the ocean .

Officials at the Carlsbad plant say they can covert two gallons of seawater into one gallon of freshwater by filtering out 99.9 % of the salt .

There are some 16,000 desalination plants on the planet , and their numbers are rising . The amount of desalted water produced around the world has more than tripled since 2000 , according to the Center for Inland Desalination Systems at the University of Texas at El Paso .

`` Desalination is growing in arid regions , '' said Tom Davis , director of the center . `` We are making progress in the USA , but the countries around the Persian Gulf are way ahead in the use of desalination , primarily because they have no alternative supplies of freshwater . ''

Israel , in an arid region with a coastline on the Mediterranean , meets half its freshwater needs through desalination . Australia , Algeria , Oman , Saudi Arabia and the United Arab Emirates also rely heavily on the ocean for their municipal water .

In the United States , desalination projects are concentrated in coastal states such as California , Florida and Texas .

Some environmentalists are wary of desalination , which consumes large amounts of energy , produces greenhouse gases and kills vital marine organisms that are sucked into intake pipes .

But proponents believe the technology offers a long-term , sustainable solution to the globe 's water shortages . One entrepreneur has even built an experimental solar desalination plant in California 's San Joaquin Valley .

`` When other freshwater sources are depleted , desalination will be our best choice , '' said Davis , a UTEP professor of engineering .

California dreaming

Within the United States , the water crisis is especially severe in California , which has been stricken by drought over the last three years .

California 's biggest source of freshwater is the snow that falls in the Sierras and other mountains , where it slowly melts into creeks and makes its way into aquifers and reservoirs . But if the planet continues to grow warmer , snow will increasingly fall as rain and will be harder to collect because it will swell creeks faster and create more flooding , said Bowles of the Water Education Foundation .

Seventeen desalination plants are being built or planned along the state 's 840-mile coastline . City officials in Santa Barbara recently voted to reactivate their desalination plant , which was built in 1991 but shut after heavy rains filled nearby reservoirs in the early 1990s . Another $ 200 million facility has been proposed for the Bay Area , although construction wo n't likely begin for several years .

`` The key question with ocean desalination is how much are you willing to pay for it ? The amount of energy required to desalt ocean water can be daunting , '' said Bowles , adding that operating costs at the Santa Barbara plant alone are estimated at $ 5 million per year .

But advocates believe the price of desalination will continue to decrease as the process improves . This will be true of the massive Carlsbad plant , said Bob Yamada , water resources manager with the San Diego County Water Authority .

`` The cost for this water will be about double what it costs us to import water into San Diego , '' Yamada said . `` However , over time we expect that the cost of desalinated water will equal , and be less than , the cost of imported water . That may take 15 or 20 years , but we expect that to occur . ''

Ultimately , experts say , municipalities will need to balance desalination projects with conservation and water from more traditional sources , such as rivers , reservoirs and recycled wastewater .

`` You ca n't get all your water from one source and have that source be hundreds of miles away , '' said Peter MacLaggan , senior vice president at Poseidon Resources Corporation , which is leading development of the Carlsbad plant .

`` When and if the drought does come , and you do n't have enough snowpack in the Sierras -- after 12 dry years the Rockies are seeing the impact of that today -- you 've got -LRB- water -RRB- sources here within the boundaries of San Diego County , '' he said .

`` We have a $ 190 billion economy in this region . It 's dependent on water to sustain that economy . So the question you need to consider , is ` What 's the cost of not having enough water ? ' ''

Drought-ravaged California turns to tech for help

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There are 16,000 desalination plants on the planet , and their numbers are rising

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A huge desalination plant is under construction in Carlsbad , California

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When completed it will be the largest such facility in the Western Hemisphere

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Water expert : Ocean water is `` a seemingly inexhaustible supply ''